DISSECTION OF THE DOG 219 



the columns of the fornix, and the anterior commissure. The ventricle com- 

 municates with the lateral ventricles through the foramen of Monro, and 

 posteriorly an aditus (aditus ad aquseductum cerebri) gives access to the 

 aqueduct of Sylvius. 



The regularity of the contour of the cavity is broken by several recesses. 

 Among these is the recess of the infundibulum (recessus infundibuli) extending 

 into the hypophysis. The optic recess (recessus opticus) is just dorsal to the 

 optic chiasma. In the base of the pineal body is a small pineal recess (recessus 

 pinealis), and above it a much more extensive suprapineal recess (recessus 

 suprapinealis). 



Commissura anterior. — The anterior commissure is a transverse band of 

 fibres readily demonstrated in the hemi-sected brain. If such a specimen be 

 examined, the commissure will be seen in section as a small oval white object, 

 immediately in front of the columns of the fornix. In the specimen from which 

 the structures overlying the thalami have been removed, a glimpse of the 

 commissure may be obtained between the slightly divergent columns of the 

 fornix. 



Commissura posterior. — The posterior commissure consists of a 

 narrow white strand crossing the brain transversely at the base of the pineal 

 body. It is best demonstrated in a median longitudinal section. 



Dissection. — This is a convenient time at which to make a complete ex- 

 amination of the mid-brain. Strip the membranes from the corpora 

 quadrigemina, aboral to the thalami in position, and from the adjacent 

 parts of the cerebellum. Be careful to preserve the small trochlear 

 nerve to be found deep down in the depression between the cerebellum 

 and the corpora quadrigemina. 



Mesencephalon. — The mid-brain is the short and narrow segment joining 

 the pons and cerebellum on the one hand with the cerebrum on the other. 

 It has a dorsal part formed by the corpora quadrigemina, and a larger ventral 

 portion consisting of the peduncles of the cerebrum. In the intact brain the 

 corpora quadrigemina are completely hidden by the cerebral hemisphere. 

 Running through the mid- brain is a passage, the aqueduct of Sylvius (aquaeduc- 

 tus cerebri [Sylvii]), which connects the third and fourth ventricles. 



Corpora quadrigemina. — These are four rounded elevations lying between 

 the thalami and cerebellum and arranged in two pairs. The anterior eleva- 

 tions (colhculi anteriores) are somewhat more clearly defined, and are closer 

 together than the posterior colhculi. These latter are separated from each 

 other by a wide and shallow groove, from which a thin thread-like frcenulum 

 veli runs into the anterior medullary velum. The two colhculi of the same 

 side are separated by a narrow but not very deep transverse groove. 



Laterally each colhculus is continued by a brachium. The brachium of the 

 anterior colhculus (brachium quadrigeminum anterius) is scarcely demonstrable, 



